Three-level converters can be converters that have three DC poles. In addition to positive and negative DC poles, they have a neutral DC pole. Examples of three-level neutral-point-clamped converters have been described in T. Brückner, S. Bernet and H. Güldner, “The Active NPC Converter and Its Loss-Balancing Control”, IEEE Transactions on Industrial Electronics, Vol. 52, No. 3, June 2005.
FIG. 1 shows an example of a main circuit of a switching branch of a three-level converter according to a known implementation. The switching branch includes six diodes D1 to D6 and six controllable semiconductor switches S1 to S6. A converter including one or more switching branches, like that of FIG. 1, can operate as a rectifier or as an inverter. The switches S1 to S6 can be controlled according to a desired modulation scheme.
When a three-level converter is set to enter a stop state, in which all the semiconductor switches of the main circuit of the converter can be turned OFF and a possible remaining current flowing in the circuit flows through diodes D1 and D2 or D3 and D4 until going to zero, it should be carried out safely in case of a fault situation. U.S. Pat. No. 7,508,640 discloses a method for fault handling in a converter circuit, in which method a fault current path is first detected and on the basis of this information a suitable fault switching sequence is then performed. In the method disclosed the switching status of each semiconductor switch is observed and the fault situation is then determined.
A possible problem related to the above solution is that the detection and determination of the fault should be performed phase-specifically. As a result, each phase of the converter should detect the fault in order to carry out the suitable fault switching sequence. This, in turn, can cause a delay to the response to the fault and possibly cause damage to the converter equipment.